1. Field of the Invention:
The invention relates to a high-temperature protective coating for austenitic materials and more particularly for alloys and components for such coatings.
2. Description of the Related Art:
High-temperature protective coatings of this type are used primarily for protecting the base material of structural elements made of heat-resistant steels and/or alloys that are used at temperatures over 600.degree. C.
These high-temperature protective coatings are intended to retard or completely suppress the effects of high-temperature corrosion caused by sulfur, oil ash, oxygen, alkaline earths and vanadium. Such high-temperature protective coatings are preferably designed to be applied directly to the base material of the structural element to be protected.
High-temperature protective coatings are especially important in structural elements in gas turbines. They are applied to the rotor blades and guide blades and to those gas turbine segments where the heat tends to build up.
For the manufacture of these structural elements, an austenitic material based on nickel, cobalt or iron is preferably used. In the manufacture of gas turbine components, nickel superalloys are primarily used as the base material.
Until now structural elements intended for gas turbines have been conventionally provided with protective coatings formed from alloys having nickel, chromium, aluminum and yttrium as their essential components. Such high-temperature protective coatings have a matrix with an aluminum-containing phase embedded in it. When a structural component provided with such a high-temperature protective coating is exposed to an operating temperature of more than 950.degree. C., the aluminum contained in the phase begins to diffuse to the surface, where it forms an aluminum oxide cove film.
A disadvantage here is that this aluminum oxide film does not have particularly good adhesion. It tends to wear off over time from corrosion, so that the resultant automatic protection for the high-temperature protective coating is lost. In the course of time, the corrosion becomes so extensive that the matrix of the high-temperature protective coating is itself attacked.
However, it has been found, all aspects considered, that structural elements of austenitic materials are best protected by such high-temperature protective coatings, and so these protective coatings cannot be dispensed with.